The present invention relates to a system for measuring an optical loss in an optical transmission line and/or an optical device like an optical switch or an optical divider utilized in an optical communication, in particular, relates to such a system for measuring an optical loss in a single mode optical fiber which is considerably thin, and/or in an optical device for a single mode optical fiber.
Two of the prior systems for measuring an optical loss in an optical fiber and/or an optical device are a cut-back system and the insertion loss measuring system.
FIGS. 1A and 1B show the principle of a prior cutback system. In those figures, the light source 1 and the light receiver 2 are connected by the optical fiber 3 of which the optical loss is to be measured. After the loss in the optical fiber 3 is measured, that optical fiber 3 is broken at the point (a) which is near the optical source 1, and the optical loss until the point (a) is measured again. Then, by comparing the optical loss between that at the point (a) of FIG. 1B and that of the long fiber 3 of FIG. 1A, the transmission loss in the optical fiber 3 can be obtained.
Generally, in order to measure the optical loss, a measuring system and a device to be measured must be coupled, and the coupling loss generated at the coupling point between that measuring system and the device to be measured is inevitable. Although that coupling loss can be estimated, that coupling loss can not be separated from the measured data, and so the coupling loss appears as an error of measuring.
With regard to that coupling error, the cut-back system has the advantage that the coupling loss between the optical fiber 3 and the light receiver 2 is small since the diameter of the light receiver 2 is extremely larger than that of the optical fiber 3. However, that cut-back system has the disadvantages that the fiber to be tested must be broken, and the measured result depends upon the fluctuation and/or the linearity characteristics of the light source and/or the light receiver, since this system is based upon the measurement of the absolute values of the loss.
Another prior art, the insertion loss measuring system is shown in FIG. 2. The sample 3 to be tested is connected to the measuring system 4 at the coupling points (c) and (d), and the difference between the loss in the sample 3 and the reference loss prepared in the measuring system 4. This insertion loss measuring system has the advantage that the loss is measured without breaking an optical fiber, but has the disadvantage that the coupling loss is still included in the measured data, and increases the error of the measuring.
By the way, it has been well known that the substitution method for measuring the loss in an electrical circuit provides the excellent accuracy. FIG. 3 shows that substitution method. In FIG. 3, the reference numeral 5 is a signal generator, 6 and 7 are switches, 8 is a signal level detector, 11 is a sample to be tested, and 12 is a reference variable attenuator. One of the sample 11 and the reference attenuator 12, is selected by the switches 6 and 7, and the variable attenuator 12 is adjusted so that the level at the level detector 8 is constant irrespective of the switching between the sample 11 and the reference attenuator 12, then, the loss in the sample 11 is measured as the same as the loss in the reference attenuator 12. The substitution method has the advantages that the non-linearity characteristics of the level detector 8, and that the measured result is free from the fluctuation of the signal generator 5 and the level detector 8.
However, the application of the substitution method to an optical communication field has some problems some of which are the stability of characteristics of an optical switch and/or an optical attenuator, and the coupling between optical fibers. In those points, as far as a multimode optical fiber which has the core diameter of 50 .mu.m is concerned, an excellent optical switch and an optical attenuator have been developed and are available on the market. Further, the coupling loss between multi-mode optical fibers is sufficiently small.
However, when a single mode optical fiber which has the diameter of less than 10 .mu.m is concerned, a satisfactory optical switch and/or an optical attenuator can not be obtained since the size of those devices is too small to produce. Although we try to produce that optical switch and optical attenuator for a single mode optical fiber, the insertion loss in the switch or the attenuator itself is large, and the characteristics of those devices are not stable.
Accordingly, the application of the substitution method to the measurement of a single mode optical fiber has been impossible.